The invention relates to a system enables the freeing of motor vehicles provided with an automatic transmission, by repeated forward-reverse operation.
As is well known, in the course of driving under conditions of limited traction, it often occurs that when starting, driven wheels spin due to high rolling resistance of the vehicle. In this case the driver tries to "rock" the vehicle from its stationary position by moving forward and backward. This can be carried out in the quickest and most advantageous way if the driver shifts the transmission alternately into forward and reverse gears.
Although an automatic transmission gear change can be carried out easily by moving the shift lever, problems may arise in respect to the fact that change of direction takes place in a state of motion of the vehicle and engine, which may result in coarse shiftings and eventually in damage to the driving system. In consideration of the fact that vehicles typically need to be started in this way rather infrequently, producers of vehicles generally are of the opinion that drivers should pay increased attention to the gear-change procedure and no special equipment is provided to assist this procedure. Considering that drivers in such situations typically may be inconsistent and inattentive, the viewpoint of the producers is regarded as rather erroneous.
Accordingly an objective of our invention is to provide a system whereby vehicles, provided with automatic transmission, can be freed by repeated forward-reverse shifting without coarse shifting. This is achieved by so controlling the transmission that, when the driver initiates a gear-change, he only begins a signal to change the direction. The actual gear change takes place in a controlled manner, when the state of motion of the vehicle and the motor are at suitable levels.
More specifically, the invention is directed to a method and apparatus for freeing a locked-in vehicle by forward and backward rocking, wherein the entire operation can be controlled semiautomatically, simply by pressing and releasing the throttle pedal. The system is provided with a master control switch, which normally disables the rocking control. When the switch is actuated, then the system is set into motion by a sensing of wheel speed equal to near zero, engine speed not significantly in excess of idle, and a depression of the throttle pedal. The automatic transmission immediately shifts into forward gear and drives the wheels in a forward direction, until the throttle pedal is released. Upon release of the throttle pedal, and return of the wheel and engine speeds to desired low levels, the system is reenabled, and can be reactivated by simply pressing again on the throttle pedal. By means of a flip-flop control in the system, the next pressing of the throttle pedal causes the automatic transmission to be shifted into reverse, so that the vehicle is rocked in the opposite direction. Alternate pressings and releasings of the throttle thus cause the vehicle to be shifted alternately into forward and reverse. The intervening requirements are that the engine and wheel speeds be allowed to return to a level at which the shifting can occur without damage to the transmission and/or drive train.
The invention will be described in detail by reference to a preferred embodiment, by the aid of the accompanying drawings.